Flash photolysis system

ABSTRACT

A pump-probe LFP system is adapted to a substantially lower energy requirement of a pump light source and a probe light source. The LFP system includes a photonic crystal fiber based probe light source, a pump light source adapted to produce light pulses with nanojoule or higher energy, a main laser source to generate a first beam portion to the probe light source and a second beam portion to the pump light source, a delay generator, computer, an optical modulator, and a detector.

FIELD OF THE INVENTION

The present invention relates to the field of laser flash photolysis andmore particularly to a flash photolysis system with improved performanceover existing flash photolysis spectrometers.

BACKGROUND OF THE INVENTION

Laser flash photolysis (LFP) is a technique utilized to study reactionmechanisms in chemical and biological processes. The technique wasintroduced in 1966 by Lindqvist at the CNRS in France and the techniquewas quickly developed by various research groups around the world. LFPwas brought about by the invention of the laser in the early 1960s. Thetechnique of LFP consists of a pulsed laser source that generates achemical species in a sample to be studied, an optical and electronicsystem capable of sensing optical changes in a sample, and a computersuitably equipped to selectively capture, process, and display the data.The optical and electronic systems constitute a fast spectrometercapable of acquiring spectra of short-lived chemical species called“intermediates”. The optical and electronic systems then record theevolution of the intermediates over time. The time resolution in suchfast spectrometer can be achieved by two primary methods.

A first method includes use of fast electronics where a readout of afast detector is digitized and recorded in real time, or when anelectronic gating is applied to the detector. The electronic gating istypically used with array-based spectrometers where the output cannot beprocessed rapidly enough to perform real time data acquisition. Bothtechniques typically utilize continuous wave (CW) or pulsed xenon arclamps as a probe light source. Due to the low intrinsic brightness andpoor collimation of a probe beam produced by the probe light source, anoptical overlap between the probe and a pump (excitation) beam takesplace over an area of approximately 1 cm², thereby placing energyrequirements on the laser pulse necessary to induce chemical changes inthe sample. The corresponding pump laser pulses typically have energy ofa few millijoules. Because of the pulse energy requirement, only alimited number of lasers, known as Q-switched lasers, can be used withthe xenon arc lamp probe light source to produce the required energy.

A second method is called optical gating or the “pump-probe” method. Inthis method, the dynamics of a chemical change of a sample is monitoredby studying a series of light pulses from a laser at different times asthe light pulses (pump beam) are passed through the sample. The probeand pump beams travel through the same volume of the sample studied. Apulse of the pump beam induces a transient chemical change in the samplewhich affects the optical properties of the sample. A spectrum of apulse of the probe beam passing through the sample is altered by thechanges made to the sample by the pump beam depending on when the probepulse arrives at the sample with respect to the pump pulse.

Where the probe beam travels in front of the pump beam, the probe beamwill only measure the sample before the excitation event. As the probebeam is delayed, it arrives at the sample simultaneously with the pumppulse, corresponding to a time zero. The delay of the probe beam isincrementally increased over a desired time interval. The correspondingchanges in the probe beam monitored by a detector are therefore assignedto particular delays (time) after the excitation event. A series ofprobe beams at various delays represents information about the dynamicsof the changes in the sample caused by the pump beam.

At each of the delays of the probe beam, two spectra of the probe beamare recorded by the detector, A first spectrum corresponds to the probebeam traveling through the sample together with the pump beam. A secondspectrum, a reference spectrum, corresponds to the probe beam sentthrough the sample without the pump beam. Usually at a particular pumpprobe delay, a series of such probe spectrum pairs are averaged in orderto obtain a sufficient signal to noise ratio. The pump beam energy insuch experimental setups is often limited to several microjoules.Therefore, in order to achieve comparable instrument sensitivity andsimilar photon flux in the excitation beam, the pump beam and the probebeam are spatially overlapped in the sample over an area less than 1mm². Generation of a probe beam that can satisfy the above requirementis possible only if a highly collimated beam such as a laser is used.

Optical gating has been successfully used with femtosecond andpicosecond lasers. The femtosecond or picosecond laser output is splitinto several parts, one of which is used to produce a probe beam withdesired wavelength specifications, usually through super-continuumgeneration or optical parametric amplification. The materials used forsuper-continuum generation are typically bulk materials—crystals such assapphire, calcium fluoride, etc. or liquids such as water, etc. Theresulting beam is then used to probe the photo-induced changes in thesample. The time resolution is realized by varying the travel pathlength of the probe beam with respect to the pump beam, which allows forextremely high temporal resolution, down to several femtoseconds.However, in order to generate a super-continuum in bulk materials suchas sapphire one needs to have laser pulses with high peak power (MegaW),which can be produced by only a limited number of lasers includingamplified femtosecond lasers. Such amplified femtosecond lasers areexpensive and have a large footprint (8-10 ft²).

Commonly owned U.S. Pat. No. 7,817,270 B2 shows a nanosecond pump-probeLFP system that is adapted to a substantially lower energy requirementof a pump light source and a probe light source. The LFP system includesa photonic crystal fiber based probe light source, a pump light sourceadapted to produce light pulses with nanojoule or higher energy, a delaygenerator, a computer, and a detector.

SUMMARY OF THE INVENTION

A LFP system for performing laser flash photolysis have beensurprisingly developed and are adapted to substantially lower the energyrequirement of a pump and probe light source. The LFP system includes aprobe light source, a pump light source adapted to produce light pulseswith nanojoule or higher level energy, a delay generator, an opticalmodulator, and a detector.

In the system for flash photolysis a main laser source generates a beamof light and a beam splitter generates a first portion beam and a secondportion beam from the beam of light. The probe light source isresponsive to the first portion beam for generating a first pulsed beamof light to travel through a sample in a flash photolysis application,wherein the probe light source is a laser energized photonic crystalfiber. The delay generator is one of an optical delay line or an opticaldelay generator to regulate a time delay between generation of the firstpulsed beam and generation of the second pulsed beam throughout theflash photolysis application. The pump light source is responsive to thesecond portion beam from the delay generator for generating a secondpulsed beam of light to travel through the sample and initiate achemical reaction in the sample. The detector receives the first pulsedbeam exiting the sample to detect a change in absorption of the firstpulsed beam in the sample caused by the second pulsed beam.

DESCRIPTION OF THE DRAWINGS

The above and other objects and advantages of the invention will becomereadily apparent to those skilled in the art from reading the followingdetailed description of the invention when considered in the light ofthe accompanying figures.

FIG. 1 shows a schematic layout of an LFP system according to anembodiment of the present invention.

FIG. 2 shows a schematic layout of an LFP system according to anotherembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The following detailed description and appended drawings describe andillustrate various exemplary embodiments of the invention. Thedescription and drawings serve to enable one skilled in the art to makeand use the invention, and are not intended to limit the scope of theinvention in any manner. In respect of the methods disclosed, the stepspresented are exemplary in nature, and thus, the order of the steps isnot necessary or critical.

Referring to the figures, each illustrates an LFP system 10 whichincludes a probe light source 12, a pump light source 14, a detector 16,and a delay generator 18. The probe light source 12 shown is a photoniccrystal fiber pumped by a laser 51. The probe light source 12 is adaptedto be focused to areas as small as several square microns. It isunderstood that the probe light source 12 may be any conventional probelight source that is adapted to be focused to areas as small as severalsquare microns such as a pulsed femtosecond laser oscillator coupled toa photonic fiber, for example.

The pump light source 14 is typically a harmonics generator or anoptical parametric oscillator pumped by the laser 51 adapted to producea collimated beam with an energy level of at least several nanojoulesand referred to as an excitation light source. The pump light source 14may also be an amplified femtosecond laser, a non-amplified femtosecondlaser oscillator, a picosecond laser, a Q-switched nanosecond laser, adye laser, a nitrogen laser, or a nanosecond microchip laser, asdesired.

The detector 16 is a broadband detector, such as a CCD basedspectrometer adapted to measure a change in the absorption of light by asample 20. The detector 16 may be further adapted to transmit ameasurement signal to a computer 23. It is understood that the detector16 may be any device adapted to measure the properties of light over aspecific portion of the electromagnetic spectrum.

The delay generator 18 is typically an optical delay generator. Thedelay generator 18 is in electrical communication with the computer 23and adapted to optically control the time delay between the probe beamproduced by the probe light source 12 and the pump beam produced by thepump light source 14. The delay generator 18 is further adapted toselectively vary the delay between the pump beam and the probe beam byany amount, though typical LFP experimentation rarely requires a delayof over 10 nanoseconds. It is understood that the delay generator 18 canbe installed before or after the pump light source 14, or before orafter the probe light source 12.

The LFP system 10 further includes a beam block 22, a plurality of lensoptics 24, 26, 28, 29, a plurality of reflective optics 32, 34, 36, 53,and a beam splitter 52. The beam block 22 is adapted to capture andabsorb electromagnetic energy such as, a beam of collimated light.

The lens optics 24, 26, 28, 29 include a first lens 24, a second lens26, a third lens 28, and a fourth lens 29. The first lens 24 is disposedin the path of the beam produced by the probe light source 12 and isadapted to focus the probe beam into the sample 20. The second lens 26is disposed in the path of the beam produced by the pump light source 14and is adapted to focus the pump beam into the sample 20. The third lens28 and the fourth lens 29 are adapted to collect and guide the beamproduced by the probe light source 12 to the detector 16. Although theLFP system 10 is shown having four lens optics 24, 26, 28, 29, it isunderstood that any number of lens optics may be used, as desired.Alternatively, curved mirrors can be utilized instead of lenses.

The reflective optics 32, 34, 36, 53 may be any conventional reflectiveoptics to direct light beams such as mirrors, for example. Thereflective optics 32, 34, 36, 53 are disposed in the path of the beamsgenerated by the probe light source 12 and the pump light source 14 toaffect the desired direction of the beams. Although the LFP system 10 isshown as having four reflective optics 32, 34, 36, 53, it is understoodthat any number of reflective optics may be used to affect the desireddirection of the beams.

The beam splitter 52 is disposed in the path of the beam generated by amain laser source 51 to reflect two portions of the main beam in desireddirections. The main beam is separated by the splitter 52 into a firstbeam portion directed to the probe light source 12 and a second beamportion. The embodiment shown in FIG. 1 directs the second beam portionto the pump light source 14. In the embodiment shown in FIG. 2, thesecond beam portion is first directed to an optical modulator 54. Themodulator 54 then provides the second beam portion to the pump lightsource 14.

In use, the probe light source 12 generates probe beam pulses that arefocused into an area of approximately 0.1 mm² in the sample 20 by thefirst lens 24 while the pump light source 14 generates pump beam pulsesthat are focused into the same area of approximately 0.1 mm² by thesecond lens 26 and having an energy-level of several nanojoules. Theprobe and pump beams are therefore caused to spatially overlap in thesample 20. The pump beams are captured by a beam block 22 after passingthrough the sample 20. The probe beam is collected by the third lens 28and the fourth lens 29 and guided into the detector 16. Reflectiveoptics 32, 34, 36 are provided to direct the beams from the lightsources 12, 14. Changes to the sample 20 and the difference in lightabsorption of the sample 20 are then measured by the detector 16 andrecorded by the computer 23.

The probe beam pulse is produced at a constant frequency ofapproximately 50 kHz. It is understood any frequency may be used, asdesired. The computer 23 controls timing of the pulses from the pumplight source 14 through the delay generator 18. The pulses from the pumplight source 14 are sent to the sample 20 so that that the pump pulseprecedes the probe pulse by a desired time interval, thereby generatinga desired pump-probe delay.

The optical modulator 54 is used to modulate the pump pulses from thepump light source 14. The modulator 54 can be an optical chopper, anacousto-optical modulator, an electro-optical modulator, etc. Themodulator 54 is controlled by the PC 23. To obtain a spectrum of theprobe beam pulse without the pump beam pulse present in the sample 20, asignal is sent to the modulator 54 that blocks the pump pulses at a rateof every other probe beam pulse.

By varying the timing of the probe beam and pulse beam with the delaygenerator 18, the delay between the pulses available for experimentationon a sample 20 by the LFP system 10 may be varied as desired, thoughtypically the interval for experimentation will be less than tennanoseconds. Additional benefits of the LFP system 10 include areduction in the overall size of the LFP system 10 to an area of lessthan approximately 4-6 ft² by including optical components such as thelens optics 24, 26, 28, 29 and the reflective optics 32, 34, 36, 53 todirect the beams, thereby minimizing the overall cost of the LFP system10 as compared to other commercially available LSP systems.

The LFP system 10, according to the embodiment of the invention shown inFIG. 2, includes the main laser source 51, the probe light source 12,the pump light source 14, the detector 16, the delay generator 18, andthe optical modulator 54. The probe light source 12 is a photoniccrystal fiber pumped by a first beam portion of the main light beamgenerated by the main laser 51. The probe light source 12 is adapted tobe focused to areas as small as several square microns. It is understoodthat the probe light source 12 may be any conventional probe lightsource that is adapted to be focused to areas as small as several squaremicrons such as a Q-switched sub-nanosecond microchip pulsed lasercoupled to a photonic fiber, for example.

The pump light source 14 is the second beam portion of the beam from themain laser 51 adapted to produce a collimated beam with an energy levelof at least several nanojoules and referred to as an excitation lightsource. The pump light source 14 may also be an amplified femtosecondlaser, a picosecond laser, a Q-switched nanosecond laser, a dye laser, anitrogen laser, a nanosecond microchip laser, a femtosecond laseroscillator, a harmonics generator, or an optical parametric oscillator,as desired.

The delay generator 18 is an optical delay line or an optical delaygenerator. The delay generator 18 is in electrical communication withthe computer 23 and adapted to optically control the time delay betweenthe probe beam produced by the probe light source 12 and the pump beamproduced by the pump light source 14. The delay generator 18 is furtheradapted to selectively vary the delay between the pump beam and theprobe beam by any amount, though typical LFP experimentation rarelyrequires a delay of over 10 nanoseconds.

The LFP system 10 further includes the beam splitter 52 and thereflective optic 53. The main beam generated by the main laser source 51impinges on the beam splitter 52 and is split into two portions. Thefirst portion beam is passed to the probe light source 12. The secondportion of the main laser source beam is directed to the reflectiveoptic 53 to redirect the second portion beam to the delay generator 18.

In use, the system 10, by varying the timing of the probe beam and pulsebeam with the delay generator 18, varies the delay between the pulsesavailable for experimentation on a sample 20 as desired, thoughtypically the interval for experimentation will be less than tennanoseconds.

From the foregoing description, one ordinarily skilled in the art caneasily ascertain the essential characteristics of this invention and,without departing from the spirit and scope thereof, can make variouschanges and modifications to the invention to adapt it to various usagesand conditions in accordance with the scope of the appended claims.

What is claimed is:
 1. A system for flash photolysis comprising: a mainlaser source for generating a beam of light; a beam splitter forgenerating a first beam portion and a second beam portion from the beamof light; a probe light source responsive to the first beam portion forgenerating a pulsed probe beam of light to travel through a sample in aflash photolysis application, wherein the probe light source is a laserenergized photonic crystal fiber; a pump light source responsive to thesecond beam portion for generating a pulsed pump beam of light to travelthrough the sample and initiate a chemical reaction in the sample; adelay generator to regulate a time delay between generation of thepulsed probe beam and generation of the pulsed pump beam throughout theflash photolysis application; and a detector to receive the pulsed probebeam exiting the sample to detect a change in absorption of the pulsedprobe beam in the sample caused by the pulsed pump beam.
 2. The systemfor flash photolysis according to claim 1 further including an opticalmodulator between said beam splitter and said delay generator formodulating the pulsed pump beam.
 3. The system for flash photolysisaccording to claim 2 wherein said optical modulator is operated to blocksome of the pump pulses to obtain a spectrum of the probe beam pulsetraveling through the sample when unpumped.
 4. The system for flashphotolysis according to claim 3 wherein said optical modulator isoperated to block the pump pulses at a rate of every other one of theprobe beam pulses to obtain a spectrum of the probe beam pulse travelingthrough the sample when unpumped.
 5. The system for flash photolysisaccording to claim 2 wherein said optical modulator is operated to passthrough some of the pump pulses to obtain a spectrum of the probe beampulse traveling through the sample when pumped.
 6. The system for flashphotolysis according to claim 5 wherein said optical modulator isoperated to pass the pump pulses at a rate of every other one of theprobe beam pulses to obtain a spectrum of the probe beam pulse travelingthrough the sample when pumped.
 7. The system for flash photolysisaccording to claim 1 wherein said delay generator is one of an opticaldelay line or an optical delay generator.
 8. A system for flashphotolysis comprising: a main laser source for generating a main beam oflight; a beam splitter for generating a first beam portion and a secondbeam portion from the main beam of light; a probe light sourceresponsive to the first beam portion for generating a pulsed probe beamof light to travel through a sample in a flash photolysis application,wherein the probe light source is a laser energized photonic crystalfiber; a pump light source responsive to the second beam portion beamfor generating a pulsed pump beam of light to travel through the sampleand initiate a chemical reaction in the sample; a delay generator beingone of an optical delay line and an optical delay generator to regulatea time delay between generation of the pulsed probe beam and generationof the pulsed pump beam throughout the flash photolysis application; anda detector to receive the pulsed probe beam exiting the sample to detecta change in absorption of the pulsed probe beam in the sample caused bythe pulsed pump beam.